Quartz-to-metal seal



' April 19, 1938. c. BOL ET AL QUARTZ-TO-METAL SEAL Filed May 26, 1936Fig.5.

lmventors Cornehs Bol Hehdricus J. Lemmens by 5. Their Attorney.

Patented Apr. 19, 1938 PATENT OFFICE QUARTZ-TO -METAL SEAL Cornelis B01and Hendricus J. Lemmens, Eindhoven, Netherlands, assignors to GeneralElectric Company, a corporation of New York Application May 26,

1936, Serial No. 81,866

In Germany August 31, 1935 3 Claims.

The present invention relates toan improved quartz-to-metal sealparticularly applicable to mercury vapor electric discharge devices ofthe type described in copending application, Serial No. 46,952, filedOctober 26, 1935, by Cornelis B01, Willem Elenbaas and Hendricus J.Lemmens.

The above-mentioned application discloses a new type of mercury vaporlamp comprising a capillary quartz envelope having an internal bore ofless than 7 millimeters and preferably of from 1 to 5 millimetersdiameter. These lamps are adapted to be operated at an extremely highspecific loading and consequent high temperatures. The loading commonlyis indicated in watts per centimeter of tube length. For example, anaircooled lamp may consume from 25 to watts per centimeter, while unitsequipped with special cooling means may utilize as high as from to 1000watts per centimeter with a voltage drop of from 100 to 1000 volts percentimeter.

As a consequence of these operating conditions and the particularconformation of the envelope high vapor pressures may be developedduring operation, pressures commonly-in excess of 10 atmospheres and, inextreme cases, even in excess of 300 atmospheres.

In view of the high temperatures encountered it is necessary to form theenvelope of a refractory transparent material of which quartz is atpresent considered the most satisfactory example.

' A principal dimculty encountered in the manufacture of lamps of thismaterial consists in the provision of a durable seal for the electrodeleadin connections which is capable of remaining gastight under theconditions of operation. One method which has already been proposed forthe solution of this difficulty comprises interposing between the quartzand the lead-in conductor one or more transition glasses having gradedexpansion' coefllcients which lie between the expansion coeflicients ofthe quartz and the metal.

The present invention increases the mechanical strength of seals of thistype and/thereby raises the permissible mercury vapor pressure byproviding an improved form for the body of transition glass employed.The nature of the improvement as well as the method by which it isproduced may' best be understood by reference to the accompanyingdrawing in which Figs. 1, 2, 'and 3 are sectional views illustrating thesuccessive steps in the sealing process; Fig. 4 shows a complete sealformed in accordance with the invention, and Fig. 5'shows a dischargedevice embodying our improved form of seal.

In carrying out the invention the end of a quartz tube i, preferably ofless than-about 7 millimeters internal diameter, is filled. as bysuction, with a stopper or plug 2 of transition glass which may bemolten when introduced. One example of a glass suitable for use withtungsten lead-in conductors is described in copending application,Serial No. 43,230, filed October 2, 1935, by Cornelis Bol, Hendricus J.Lemmens, and Gottfried B. Jonas, and comprises approximately thefollowing composition:

Per cent SiO2 .1 88.43 B203 8.4 A1203 2.9 CaO .4

The plug should be of such length that it is in contact with theinterior of the quartz tube for a distance at least one and a half timesthe wall thickness of the latter and preferably from 2 to 4 times suchthickness.

The plug 2, whlchshould be maintained at a sufficiently high temperatureto preserve its plasticity is subjected to pressure at the inner endthereof in the direction indicated by the arrow in Fig. 2. Ordinarilythis will consist of air pressure provided by the glass blower andshould be of such nature as to cause a cup-like depression 4 to beformed in the inner end surface of the plug. For the purposes of thepresent invention, it is desired that the walls of the depressioncomprise a tapered rim of gradually diminishing thickness fusedexteriorly' to the quartz tube.

. Preparatory to sealing a lead-in conductor 5 (of tungsten or similarrefractory metal) into the structure so far described, the former isembedded in a second plug or solidcylinder 6 of a transition glass whichmay be of the same composition as the plug 2 or of a slightly differentcomposition to give a more satisfactorily graded seal.

Thereafter, by continuing the application of heat and pressure to theplug 2 a central aperture 3 is formed completely therethrough of suchsize as to match approximately the external diameter of the cylinder 6.The latter then is inserted in the aperture 3 and heat applied untilfusion of the two glasses takes place. We have found that best resultsare obtained when the cylinder 6 is so positioned that it protrudes veryslightly from the stopper 2 at its inner end, thus leaving arounded'bead or knob surrounding the conductor 5 at the region where it projectsinto the bore of the quartz tube. i

-meters with a wall thickness of about the same magnitude. Theelectrodes 5 are spaced about 20 millimeters and are shown as beingsurrounded adjacent the lead-in connections with small globules ofmercury 8 adapted to be at least partially vaporized during operation ofthe lamp. A small quantity of a readily ionizable gas, for example, neonor argon, is also enclosed in the envelope to facilitate initiation ofthe discharge.

Tests made on lamps constructed as described in the foregoing haveresulted in a surprisingly small number of rejections and indicate thatthe particular form specified produces a minimum of strain between theedges of the transition glass and the quartz tube. There is little or noapparent tendency for the plug to crack free from the quartz surface atthe superatmospheric pressures at which the seals have'been used, evenat pressures as high as 300 atmospheres.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A quartz-to-metal seal comprising a quartz tube having an inside boreon the order of a few millimeters in diameter and a wall thickness notsubstantially greater than the diameter of the bore, a plug oftransition glass projecting into the end of said tube and in sealingengagement with the interior wall thereof for a distance in excess of1.5 times the wall thickness of the latter. and a metal lead-in wirefused into the plug, said plug terminating at its inner end in a hollowportion having walls of gradually diminishing thickness fused to theinterior surface of said tube.

2. A quartz-to-metal seal comprising a quartz tube, a sealing plug oftransition glass terminating within the tube in a hollow portion havingwalls of gradually diminishing thickness fused to the interior wallsurface of the tube, a solid cylinder of a second transition glasspassing axially through said plug and slightly protruding therefrom atthe inner end thereof and a metal lead-in conductor embedded in saidcylinder and projecting into the bore of said quartz tube.

3. An envelope for an electric discharge device comprising a tubecomposed of a vitreous material which has the characteristics of quartz,plugs of transition glass fused into the ends of. said tube and lead-inconductors sealed through the plugs, each of said .plugs terminating atits inner end in a hollow portion having walls of gradually diminishingthickness fused to the interior surface of said tube.

' I CORNELIS BOL.

HENDRICUS J. LEMMENS.

